Riley's first appearance in Diamond, Pearl, and Platinum is at Iron Island. He helps the player to get through the deep mines on the island, and assists in the defeat of some Team Galacticgrunts. After the Grunts have been defeated, Riley offers the player a Pokémon Egg that eventually hatches into a Riolu. In Platinum, he will give the player HM04 (Strength) before he or she enters Iron Island's cave.

Apparently, Byron had previously asked Riley to become a Gym Leader, though Riley politely refused, saying that Byron's son Roark should be considered first.

Riley mentioned to the Team Galactic members that Iron Island actually belonged to him, suggesting he may be the owner of the mines and someone else reopened the island as a place for wild Pokémon to live as well as a place for people to train.

Quotes

Iron Island

"...You are? OK, you're <player>. I'm Riley. I'm a Trainer, yes, and it is a bit odd that I would be training here. If you'd like, we could team up. The wild Pokémon here are restless. I'll heal up your Pokémon if they get hurt."

"Ha ha! I'd have to agree with you. People training here would be those with too much time on their hands."

"This used to be a fairly thriving ore mine once upon a time. I guess the steel ore's played out. There's only Pokémon here now."

"Have you met Byron? The Leader of Canalave Gym? He's asked me to become a Gym Leader, but there's his son Roark to consider."

"Imagine what it would be like for a Steel-type Pokémon user to battle a Steel type. Would that be like battling yourself, I wonder? I mean, it's a lot harder to defeat one's self than anyone else."

"What's going on here, though? The native Pokémon are restless... I don't like this... Let's hurry."

"I get it now. You're responsible for agitating the Pokémon. I don't care what good reason you may have, but I don't want you bringing any trouble into my mine."

"To share in all the joys and all the sorrows... This is how all the Trainers and Pokémon live in Sinnoh! I won't allow anyone to deny that code of ethics! Come on, <player>! This is one battle we cannot lose!"

"Thanks. You really helped me out. Listen, I have a Pokémon Egg here. Would you take it with you?"

Yes (open party slot): "Thanks! I'd like you to show the Pokémon that's inside the Egg all sorts of new places. Spending time with you was very enjoyable. I intend to test my skills in other places from now on. Let's meet again! Take care on the way out."

In Saving the World From Ruins!, it was revealed that Riley is training to be an Aura Guardian with his Lucario and as such he is able to use Aura effectively. He was then shown to be an advanced user of Aura.

After Team Galactic planted bombs under Iron Island, Riley and his Lucario trapped the explosion using their Aura and sent it away from the island, foiling Team Galactic's plan. Later he decided to help Professor Carolina in the study of the island so that he could get to know it better.

In the manga

Riley later appears in Grumpy Gliscor where he saves Diamond and Sailor Eldritch from crashing into a large boulder with his Lucario. He reveals that he is there to train Diamond in increasing his battling skills on Byron's request. Once they arrive on Iron Island, they have a test battle to see just where Diamond's skills are lacking in. After defeating Diamond, Riley deduces that his weakness is the lack of speed and takes him to cave in order to train.

After three days of training, Riley reveals Diamond's next task: to retrieve a Pokémon Egg. After finding the boss of the cave, a Steelix, injured from a battle, they realize that someone must have attacked it and go searching for them. As they search, Riley is attacked by a Pokémon and has his and Lucario's face covered with honey, blinding them. The attackers are revealed to be two Roughnecks who were hired by Team Galactic to make a mess on the island and have stolen the Egg. With their technique that can prevent Aura users from finding them, Riley and Lucario are helpless to stop them but Diamond steps in their place. Diamond manages to defeat them without harming the Egg and with the ability to sense Aura again, Riley and Lucario finish the two off. The Egg hatches into a Riolu and Riley decides that Diamond has finally passed his test. After Diamond reveals that someone was targeting him, Riley realizes someone is spying on them. After knocking down a flying camera machine, Riley checks the machine and feels a malicious Aura emanating from it. Riley and Diamond then say their goodbyes.

Later, Riley appears at the Spear Pillar, having tracked the Aura on the camera machine to this location. Although the Aura he saw was that of the Advanced level Grunt, it turns out there was an even darker one coming from deeper inside: the Aura belonging to Sird. Riley attacks Sird and the other Commanders but they manage to escape.

Riley later appears at the Battle Frontier where he presents the machine to Palmer in hopes of being able to track Sird since the Aura he used to track her before had disappeared. Palmer then gives the machine to Thorton in hopes of his technological expertise will be of some help.

After Thorton finishes fixing the machine, Riley uses his Aura to lead everyone to where the owner is. He—along with Palmer, Platinum, Cheryl, and Mira—all set off to the Distortion World. Eventually, they manage to track the owner of the machine, the Advanced level Grunt. The Grunt, who had escaped from Team Galactic out of fear, takes them to the Distortion World, under Platinum's request.

Upon entering the Distortion World, everyone is separated due to the dimension's strange gravity. While the others land safely, Riley lands onto a floating island and is left alone. Later, he manages to meet up again with the others after the Legendary Pokémon get into a huge brawl.

He rescues Marley from getting crushed by a falling Regigigas that was put to sleep by Darkrai. When Charon tries to escape after getting his flying machine broken, Riley is left behind along with the others while Diamond, Pearl, and Platinum chase after Charon.

After Charon's defeat, Riley and everyone else who was left behind manages to escape unharmed. When Riley ponders where Darkrai went off to when the legendary Pokémon all begin to leave, the Advanced level Grunt shows it is flying to parts unknown.

Pokémon

Lucario is Riley's only known Pokémon. He trained it while on Iron Island. It rescued Hareta from B-2's Graveler. It fought bravely against his Drapion later on and won with the teamwork of Hareta's Riolu after it hatched.

Trivia

Of all the potential partners in the Battle Tower, Riley has the largest pool of Pokémon to choose from.

Riley is the only human in the games confirmed to be able to use Aura, as displayed in his intro animation and through a line of in-battle speech where he mentions feeling the player's Aura. In the anime, he is one of two known and currently living human users of Aura, the other being Ash.